Why does iced tea foam?

Answer 1

ice cream is a type of foam also, with thickening agents to maintain its consistency when the soda is poured onto the ice cream the thickening agent traps and locks the CO2 bubbles in the soda creating a thick layer of foam.

Answer 2

The carbonic acid in the soda water comes out of solution due partially to mechanical agitation (pouring), just like it does when you pour soda into an empty glass, and partially because the small particles of stuff within the ice cream (tiny ice crystals, blobs of milkfat, etc) provide nucleation sites which help bubbles to form at lower internal pressures. The reason the foam lasts so much longer with ice cream is that the milkfat and emulsifiers present in milk make the skin of the bubbles much "tougher" than pure water, because the additional components form multiple layers... sort of like plywood.

Answer 3

It's carbonated water and there is less sugar to hold it down.

Answer 4

Yes.